Here is what we know right now about the second mass shooting at ft. Hood in five years. One of the largest army posts in America. This time the shooter identified as an Iraqi war veteran being treated for mental illness. His name is army specialist Ivan Lopez. He served four months in Iraq in 2011 and had been seeking treatment for posttraumatic stress. Investigators now focusing on whether an argument on the army post triggered the shooting. He was wearing fatigues. In the end he was face to face with a female police officer. She stood her ground. That's when he shot himself. To the scene of the shooting. Alex Perez is live at ft. Hood. Good morning, Alex. Reporter: Good morning, George. This shooting bringing back chilling memories for those here back in 2009. We are told the gunman worked as a military truck driver and that he had not yet been diagnosed with PTSD. This was the loudspeaker announcement that blared through ft. Hood late Wednesday afternoon warning the base's residents to take shelter. I heard gunshots. At that point I was scared. Reporter: A soldier now identified as specialist Ivan Lopez had opened fire just after 4:25 P.M. Reportedly after an argument on the base. It is believed that he walked into one of the unit buildings, opened fire, got into a vehicle, fired from a vehicle, got out of the vehicle, walked into another building and opened fire again. Reporter: Those two buildings, a medical brigade unit and the transportation battalion. At that point we began to witness soldiers from across the street at the transportation unit fleeing. There is a barbed wire fence about seven fight and they were leaping over it. Reporter: Dressed in army ma figs he uses a handgun to kill three people. 16 others are injured and rushed to area hospitals. They've had a variety of injuries we've been treating ranging from gunshot wounds that involved extremities, abdomen, chest and neck. Reporter: At 4:45 P.M. About 20 minutes after the rampage began, the ordeal comes to a violent end. Lopez taking his own life after a female police officer confronts him in a parking lot. He was approaching her at about 20 feet. Put his hands up, then reached under his jacket, pulled out the nine mill and she pulled out her weapon and then she engaged and then he put the weapon to his head and he died of a self-inflicted gunshot wound. Reporter: President Obama speaking to the nation as scant information trickled in. I want to just assure all of us that we're going to get to the bottom of exactly what happened. Reporter: Finally at 9:52 A.M. The base is given the all clear but fear remains. Just five years ago, ft. Hood was the scene of the worst mass shooting on a military base in U.S. History when army psychiatrist nadil Hasan killed 13 people and wounded 30 others. And it was also a female officer who stopped the gunman back in that shooting in 2009. We are expecting to learn more about how exactly things unfolded here during a press briefing later this afternoon.

This transcript has been automatically generated and may not be 100% accurate.

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